1. Field of the Invention
The present invention relates to a method for manufacturing an electroluminescent (hereinafter, electroluminescent is abbreviated as EL in some cases) element which is excellent in manufacturing efficiency.
2. Description of the Related Art
In EL elements, holes and electrons injected from facing electrodes bond in a light emitting layer, excites a fluorescent substance in the light emitting layer by the energy, and emit light in color corresponding to the fluorescent substance, therefore, EL elements attract attention as a sheet display element of self light emitting type. Among them, organic thin film EL displays using an organic substance as a light emitting material, have high light emitting efficiency such as realizing of emission of high brightness even if the applied voltage is slightly less than 10 V, and the like, are capable of emitting light with a simple element structure, and there are expectations of application thereof to advertisement displays on which a specific pattern is emitted and displayed and to other low cost simple displays.
As methods for manufacturing such organic EL elements, electrodes or divisional coating of the organic EL layers which show different light emitting colors is necessary. For the low molecular weight materials, method in which materials having different light emitting colors are vapor deposited via a shadow mask, and for the high molecular weight materials, methods in which divisional coating is done by the ink jet method are mainly used.
For example, as described in the Japanese Patent Application Laid Open (JP-A) No. 2000-323276, there is a method in which, by applying an oxygen plasma treatment and CF4 plasma treatment consecutively onto the first electrode layer comprising patterning assisting layer in between the pixels, the electrode layer is made hydrophilic, and the patterning assisting layer (organic) part is made water repellent. And the light emitting layer is formed by the ink discharged from the nozzles of the ink jet apparatus. In this case, it is very important that the patterning assisting layer has water repellency because the above mentioned ink avoids the patterning assisting layer and impacts on the first electrode layer which is a hydrophilic part. However, in this method, when the hole injecting layer is needed to be formed before forming the light emitting layer, since the patterning assisting layer has water repellency, the hole injecting layer is also needed to be formed by the ink jet method. And when a procedure of forming the hole injecting layer on the entire surface is needed, the water repellency of the patterning assisting layer is lost so that forming of the light emitting layer by the ink jet method is difficult in some cases.
Moreover, for high molecular weight materials, method for manufacturing is simpler than the low molecular weight materials, but it is inferior, by far, to the low molecular weight materials in terms of the light emitting efficiency. One reason for the above is that a procedure of improving the efficiency generally used for the low molecular weight materials, which the materials of different band gap energies are laminated and the injected carriers are spatially trapped, is not used for the high molecular weight materials. That is because the high molecular weight materials are solvent soluble, it is necessary that the polarities of the solvent used for each layer differ widely in order to laminate different materials and selectivity of material is extremely narrow.
For method for manufacturing a structure effective for trapping the carries in such high molecular weight materials, a structure is effective which controls the electrical properties and traps the carriers spatially by laminating insulating material, semiconductor material, and conductor material layer-by-layer by forming very thin films by self-assembling.
For example, as described in (JP-A) No.2001-516939, there is a report for improving the light emitting efficiency by the carrier trapping effect with a structure wherein the electrical properties are controlled spatially by laminating the hole injecting layer using different materials by layer-by-layer self-assembling method. Moreover, layers can be formed by dipping a substrate repeatedly into aqueous solutions of high molecular weight materials with different electric charges so that the selectivity of the material is wide. However, in this method, films cannot be formed selectively on the substrate and films are formed on the entire surface of the substrate. Therefore, the water repellency of the patterning assisting layer is lost, that makes it difficult to form the light emitting layer by the ink jet method.
In addition, though formation of the organic light emitting layer by the ink jet method is still an effective procedure, formation of all the layers, which the organic EL layer is formed, by the ink jet method leads to problems such as the deterioration of through put and decrease of the yield.